Different Ways to Flatten a Wood Slab (Best Method?)

Discover the best methods to flatten a wood slab and achieve flat surfaces. I delve into the techniques involving router sleds, planers, CNC, and edge alignment, sharing insights on transforming rough wood slabs into flawlessly flat masterpieces.

If you happen to have a chainsaw milled slab in your possession or have recently acquired one, and you want to achieve that desirable flat surface, there are a few ways to do it – you can use a planer, a CNC machine, or a DIY Router Sled. Flat live edge slabs offer an incredible combination of strength, stability, and durability, making them an excellent choice for various applications such as kitchen and dining room tables, coffee tables, cutting boards, serving trays, and other captivating decorative pieces and furniture.

Before you start flattening the slab, it’s important to ensure that it’s properly dried for your specific climate. This step is crucial and you shouldn’t bypass it. Working with a stable and reliable material will prevent any further warping and cracking, making sure you will not need to flatten the slab again.

Now let’s get to the actual flattening. In this blog post, I will guide you through the various methods that can be employed to flatten a slab effectively, preparing it for the subsequent stages of sanding and finishing.

Table of Contents

  1. General Questions
    1. What to Consider Before Flattening a Slab
    2. How Much Material is Removed from Flattening a Slab?
  2. Different Ways to Flatten a Wood Slab
    1. Planer
    2. CNC Machine
    3. Slab Flattening Jig (Router Sled)
  3. How to Flatten a Wood Slab – Tips
  4. Frequently Asked Questions

What to Consider Before Flattening a Slab

There are several key considerations to keep in mind before flattening a wood slab. Each of these factors plays a crucial role in determining the most suitable approach, ensuring optimal results for your woodworking project.

  1.  Size of the Slab – The size of the slab plays a crucial role in determining the flattening method or technique to be used. Smaller slabs can often be effectively flattened using a planer or jointer. These tools offer sufficient capacity to achieve the desired level surface. On the other hand, larger slabs may require more advanced equipment like a CNC machine or a custom router sled to ensure successful flattening.
  2. Dryness of the Slab – Proper drying is essential to prevent potential issues such as warping and twisting. Typically, wood slabs require a drying period of 2-3 years, although this timeframe can vary based on factors such as slab thickness and wood type. Also, different sections of the slab may dry at different rates. It is a long process that takes time. 
    Various methods can be employed for drying wood slabs, including air drying, kiln drying, and dehumidifier drying.
  3. Resultant Thickness of the Slab – It is important to consider the following factors if you want to end up with an exactly thick slab. As the slab size increases, more material needs to be removed to get to the low spots and achieve a perfectly flat slab. Similarly, slabs with greater degrees of twist or distortion will require more extensive material removal.

There are a few ways how to mitigate these challenges. You may consider cutting the slab to a manageable size before flattening it or opting to rip the slab in the middle to end up with smaller pieces. Reducing the overall surface area will result in less waste. Although, that depends on the type of the project and the result you want to achieve.

Oak Slab Bar Table
Bar table from Oak

How Much Material is Removed from Flattening a Slab?

When it comes to flattening a slab, the amount of material that needs to be removed can vary. It depends on a few key factors.

  • Firstly, the initial condition of the slab plays a role. If it has significant imperfections or twists, more material may need to be taken off.
  • Secondly, the desired levelness is important. How flat do you want the slab to be? This will determine the amount of material removal required.
  • Lastly, the size of the slab matters too. Larger slabs generally require more material to be taken off to achieve a flat surface. 

Take into account that you will need to remove a lot of material even for relatively flat slabs. Typically, for a relatively flat slab, you will be removing a few millimeters from the thickness. On larger slabs that are more twisted, you might be removing up to a few centimeters from the thickness. Generally, when flattening a slab, you can be removing from a few millimeters up to a few centimeters from its thickness (1/8” – 1”). 

If you want to end up with a specifically thick slab you should add up to 2 centimeters (up to 1 inch) to the thickness of the slab to start with.

Different Ways to Flatten a Wood Slab

1. Planer

Planer for Flattening Slabs

A planer is a powerful machine designed to remove material from the surface of wood, resulting in a smooth and flat finish. It provides a superior finish with minimal additional sanding required. Planers are known for their speed, making them a time-saving option for slab flattening.


  • Superior finish: A planer delivers a smooth surface, minimizing the need for extensive sanding.
  • Time-efficient: The high speed of a planer enables quick material removal, saving a lot of time.
  • Cost-effective: Planing at a commercial shop can be a cost-effective option for flattening slabs.
  • Easy to use: Simply feed the slab through the planer, alternating sides, and monitor progress using chalk grids.


  • Flatness requirement: The initial slab should be relatively flat for optimal results, as planing cannot correct severe curves or twists.
  • Size limitations: Large live edge slabs may not fit on a standard or even a large jointer.
  • Potential tear-out: Depending on the wood grain and direction, there is a risk of tear-out during planing, requiring extra caution.
  • Price: Large planers are quite costly (tens of thousands of dollars), making them unaffordable for the majority of users.

When using a planer it is essential to begin with a relatively flat slab and consider the size of the slab and the potential for tear-out during the planing process. When a curved board is fed into a planer, it will come out thinner but still curved. The same principle applies to planing twisted and warped slabs. Despite some of the drawbacks, a planer is the number one choice for flattening large wooden slabs.

2. CNC Machine

CNC Machine for Slab Flattening
Source: WoodMizer

A CNC machine is a powerful tool for flattening wooden slabs. It operates as a computer-controlled router with a flat surfacing bit, moving back and forth, allowing for precise and automated material removal. Typically, you can surface one side of the slab on the CNC machine then flip it and pass it either through a planer or again use the CNC machine until the entire slab is flat.


  • Versatile and precise: A CNC machine can flatten slabs with complex shapes and contours, offering high precision in surface flattening.
  • Suitable for twisted slabs: When slabs are too twisted to be planed, a CNC machine can effectively flatten them, acting as a large jointer.
  • Customizable design options: CNC machines can be programmed to create intricate patterns, textures, or custom designs on the slab surface.
  • Cost: While CNC machines are a significant investment, they can be more cost-effective for long-term use compared to outsourcing flattening services.


  • Surface finish considerations: The finish achieved by a CNC machine may not be as smooth as that obtained with a planer or sander. It can result in small areas of tearout and leave grid lines that require additional sanding.
  • Time-consuming process: Flattening with a CNC machine can be a slower process, requiring multiple passes to achieve the desired flatness. It may take an hour or more of machine time, depending on the slab size and complexity.
  • Initial cost: CNC machines are substantial investments, with prices varying depending on the size and capabilities of the machine. This can be a significant consideration for individuals or small-scale woodworkers.

When comparing a CNC machine to a planer, it’s important to note the differences in finish quality, time requirements, and upfront costs. While a CNC machine offers versatility and precision, it may require more sanding time and take longer to achieve a perfectly flat surface. However, it provides an excellent option for addressing twisted slabs that cannot be planed effectively and typically is cheaper than a planer.

3. Slab Flattening Jig (Router Sled)

How to use a slab flattening jig

When it comes to flattening wooden slabs, one effective method that stands out is the use of a slab flattening jig, also known as a router sled. This tool allows you to level the surface of the slab with precision and ease. It is an affordable alternative to large planers or CNC machines. A slab flattening jig basically behaves like a CNC machine. You can easily build it at home, using MDF or plywood, or get one from a shop (this is a great router sled – Woodhaven 51). 

One of the main benefits of a router sled is that you can build it to any size you want. This way, you will be able to flatten slabs that won’t fit into a planer or a CNC machine.


  • Cost-effective option: Slab flattening jigs are significantly more affordable compared to large planers or CNC machines.
  • Customizable and scalable: You can easily adjust the size of the router sled to accommodate slabs of various dimensions, even exceeding the capacity of some planers.
  • Portability and storage: The compact nature of a router sled enables convenient storage and easy transportation, making it suitable for smaller workshops or on-site projects.
  • Versatility: With a router sled, you have the flexibility to flatten slabs of different shapes and sizes.


  • Surface finish considerations: Similar to a CNC machine, using a router sled can leave grid lines on the surface of the slab, requiring additional sanding to achieve a smooth finish.
  • Messy operation: The process of using a router sled generates a considerable amount of sawdust and debris. Consider connecting and effective dust collection.
  • Precision limitations: Depending on the sturdiness of your sled, there is a possibility of slight sagging near the center.

While it may require more sanding to eliminate grid lines and has some precision limitations, a router sled provides a practical and affordable solution for achieving a flat surface on wood slabs. Having one in the workshop definitely makes sense and it is so easy to build it.

RELATED: How to Make a Slab Flattening Jig

The video below shows how to build a slab flattening jig (router sled)

How to Flatten a Wood Slab - Tips

Here are a few tips to help you if you want to flatten a live edge slab:

  • Before starting the flattening process, evaluate the condition of the slab. Identify any major imperfections, twists, or high spots that need to be addressed.
  • Ensure the slab is securely positioned and supported during the flattening process. This will prevent unnecessary movement or vibrations that could affect the accuracy of the flattening.
  • When using tools like a planer or jointer, make multiple passes instead of removing a large amount of material in a single pass. Gradual passes reduce the risk of tear-out or uneven surfaces.
  • If the slab is twisted a lot, you can add shims underneath the slab to level it during the flattening of the first side. This will result in less waste, ending up with a thicker slab.
  • Mark a grid on the slab with a pencil or chalk to track the progress. 
  • When using a flattening jig try to run the router with the grain (not across the grain). You will end up with less tear-out and fewer marks on the surface.

Frequently Asked Questions

What is a Live Edge Slab?

A live edge slab refers to a type of wood slab that retains the natural, irregular edge of the tree it was cut from. Unlike traditional lumber, which is processed to have straight and smooth edges, live edge slabs showcase the unique and organic shape of the tree, adding character and visual appeal to woodworking projects.

Why Are Wood Slabs Expensive?

The higher cost of wood slabs can be attributed to a few factors. Firstly, it takes considerable time and effort to find and select premium slabs from mature trees. Additionally, the process of carefully drying and treating the slabs to ensure their durability and quality is labor-intensive. Finally, the unique and appealing characteristics of wood slabs, such as their natural edges and captivating grain patterns, contribute to their increased demand and subsequently higher prices in the market.

Should I Debark Wood Slabs?

The decision to debark wood slabs ultimately depends on your personal preference and the requirements of your project. However, it is generally recommended to debark wood slabs.

Here’s why: Leaving the bark intact may result in it gradually falling off over time. Additionally, debarking allows you to round and sand the live edges, reducing any sharpness. Furthermore, removing the bark helps eliminate potential insects or bugs that may be residing between the bark and the sapwood. If you do prefer to keep the bark on live edge slabs, using epoxy resin is an effective way to seal it and maintain its appearance.

What are the Most Common Wood Slab Defects?

The most common defects found in wood slabs include slab warping, slab cracking, holes in wood, and soft spots.

Slab warping refers to the distortion of the wood slab’s shape, often caused by uneven drying or changes in moisture content. Slab cracking occurs when the wood undergoes stress due to moisture changes, resulting in visible cracks. Holes in wood can be caused by insect activity or natural wood defects. Soft spots indicate areas of the slab that have weakened or decayed, compromising the overall integrity.

What Router Bit for Flattening Wood Slabs?

When it comes to flattening wood slabs using a router, a straight bit or a leveling (surfacing) bit is commonly used.

Straight bits with a large diameter and flat bottom are ideal for initial leveling and removing excess material. On the other hand, leveling bits are specifically designed for slab flattening, featuring a wide cutting diameter and multiple cutting edges. These bits are effective in achieving smooth and flat surfaces. It’s essential to choose a router bit suitable for the size of your wood slab and consider the desired outcome of your flattening process.

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About the author, Lukas
About the author, Lukas

Meet the creator of AllFlavor Workshop! As a passionate DIYer and woodworking enthusiast, Lukas is always looking for ways to make things himself rather than buying them off the shelf. With a keen eye for design and a knack for working with wood, Lukas enjoys sharing his craft with others and helping them discover the joy of building. Whether you're an experienced woodworker or a novice looking to try your hand at a new hobby, you're sure to find plenty of inspiration and tips on AllFlavor Workshop.